The invention is directed to a timer modulator for use with constant off time current mode controllers. In particular, the invention is directed to a circuit for modulating the off time of the current mode controller in response to an overabundance of current.
Current mode control as used in power supply systems operate by monitoring the amount of current flowing in a switching element such as a transistor driving a transformer. When the current flowing in the inductance of that transformer reaches an appropriate amount as demanded by a control signal, the controller turns off the switching element allowing the energy stored in the primary of the transformer to be transferred to the secondary and ultimately to the output of the power supply. An error amplifier compares the output voltage to a fixed reference and produces the control signal that determines the required amount of current to be stored in the primary. The amount of time that the switching element stays off depends on what type of current mode scheme is being used.
The scheme associated with the present invention is classified as the constant off mode. When the current reaches the required peak value, the switching element turns off for a fixed time as determined by a timing circuit. Timing components are selected by the power supply designer to ensure that the switch remains off for the appropriate time.
Under output short circuit conditions though, a problem exists. With the output shorted, the error amp will ask for maximum peak current. Because of the finite propagation delays inherent in the control circuitry, a certain amount of time is required to turn off once the maximum peak current is detected. Thus, more current than required flows in the primary. Since the time that the output switches off is fixed there is no way to reduce the duty cycle under short circuit conditions. This can cause the transformer to saturate and result in possible destruction of the switching element.
It is an objective of this invention to monitor the current in a switching element and to increase the amount of off time in a constant off time current mode controller as a function of the amount of over current during overload conditions.